1. Field of the Invention
The present invention relates to a Mn-Zn ferrite used for a magnetic core of a transformer, noise filter or similar device.
2. Description of the Related Art
In general, a high-quality soft ferrite is produced by using an iron oxide with an extremely low impurity content. Table 1 shows examples of the impurity content of several high-purity oxides for high-quality soft ferrite.
The rows marked A and B in Table 1 show examples of two high-purity iron oxides produced by a crystal refining method. In the crystal refining method, the crystals of iron sulfate or iron chloride are crystallized from an aqueous solution thereof, and these crystals are oxidized into an iron oxide. In this method, however, the impurities are not reduced sufficiently by one occurrence of crystallization because some of the impurities still are included in the crystals. Thus, the crystals are dissolved in water again, and crystallized from the solution a second time. These treatments are repeated several times, thereby further reducing the impurity content of the crystals. Utilizing this crystal refining method, a high-purity iron oxide containing not more than about 0.005 wt. % of P is obtained, but the production process is too complicated and the production cost is too high. With this crystal refining method, the Mn content of the iron oxide is less than about 0.1 wt. %.
TABLE 1 __________________________________________________________________________ Examples of high-purity iron oxides (ppm) SiO.sub.2 P Cu Ni Cr Co V Al Mn Zn Remarks __________________________________________________________________________ A 40 23 6 -- &lt;1 -- -- 11 660 340 Crystal Refining Method B 85 27 2 -- 72 -- -- 19 540 13 Crystal Refining Method C 31 15 3 32 &lt;1 12 &lt;1 &lt;1 503 3 Laid-Open Publi- cation No. 235221/1987 D 51 20 5 40 3 16 &lt;1 &lt;1 550 5 Laid-Open Publi- cation No. 235221/1987 __________________________________________________________________________
Examples C and D in Table 1 show iron oxides described in Japanese Patent Application Laid-Open Publication No. 235221/1987, which contain not more than 0.005 wt. % of P. In this method, however, iron is electrolyzed, an anolyte is separated, floc is added thereto, and the resulting solution is filtered. The filtrate then is alkalified and oxidized, and the precipitate thereof is recovered and heated. As is evident from the above description, this process is also complicated and the production cost thereof is very high. In this method, the Mn content of the iron oxide is less than 0.1 wt. %.
Among the various impurities in a typical iron oxide, there are some which impair the ferrite properties of soft ferrite, and others which do not. However, in the conventional processes described above where all types of impurities are simultaneously removed, the production process is overly complicated and too expensive. Conventionally, it has not been possible to control the removal of specific impurities to produce an iron oxide having a desired purity level of one impurity, and a different purity level for another impurity.
Accordingly, it is an object of the present invention to provide an iron oxide in which the harmful impurities for ferrite properties have been selectively removed.
It is another object of the invention to provide an iron oxide for high quality soft ferrite which can be made by a simple process.
A further object of the invention is to provide high-quality soft ferrite at a reduced cost.